Resistance of interconnect lines (such as source/drain interconnect lines) plays a significant role in controlling the speed of ultra large-scale integration (ULSI) devices. A self-aligned silicide process (typically referred to as a salicide process) is often used to reduce the resistance of interconnect lines. A salicide process employs metal layer deposition and subsequent annealing to cause a reaction between the metal and underlying silicon. This reaction consumes the metal and some of the silicon, forming a low resistivity metal silicide layer in their place.
Nickel is an attractive candidate for use in a salicide process. Nickel can react with silicon to form low resistivity nickel mono-silicide as the salicidation product. Nickel mono-silicide can be formed during a one-step, low-temperature annealing process. Nickel mono-silicide has low silicon consumption during salicidation. In addition, nickel mono-silicide sheet resistance does not depend upon device linewidth. Thus, a well-controlled nickel salicide process could be used to form low resistivity gate electrodes and source/drain contacts in an integrated circuit device.
One prior art nickel silicidation process is described by S. R. Das et al., Mat. Res. Soc. Symp. Proc., 427, 541 (1996). This process comprises depositing nickel onto polycrystalline silicon (poly-silicon), and subsequently annealing the sample. During annealing, nickel reacts with silicon to form nickel silicide. However, the reaction product is not a uniform thin nickel silicide film on the surface of the poly-silicon layer. Nickel enhances poly-silicon grain growth near the surface of the poly-silicon layer. Rapid poly-silicon grain growth causes layer inversion, wherein some poly-silicon grows on top of the nickel silicide layer. The resulting nickel silicide layer is not uniform and flat.
A second prior art nickel silicidation process is described by S. Nygren et al., Appl. Surf. Sci., 53, 57 (1990). This process comprises depositing nickel on single crystal silicon, and subsequently annealing the sample. During annealing, nickel reacts with silicon to form nickel silicide. However, the thin nickel silicide film agglomerates during the annealing process, forming a discontinuous island structure on the surface of the silicon substrate. Some nickel mono-silicide also transforms to higher resistivity nickel di-silicide.
A third prior art process is described by W. T. Sun et al., IEEE Trans. Elec. Dev., 45, 9 (1998). This process comprises: (1) implanting poly-silicon with nitrogen ions; (2) depositing cobalt on poly-silicon; and (3) annealing the sample twice. During annealing, cobalt reacts with silicon to form cobalt silicide. Nitrogen ion implantation inhibits agglomeration and inhibits transformation from cobalt mono-silicide to lower resistivity cobalt di-silicide. Unfortunately, this process also increases cobalt di-silicide resistivity and requires a two step annealing process.
A fourth prior art process is described by L. W. Cheng et al., Thin Solid Films, 355–356, 412 (1999). In this process, a single crystal silicon substrate is implanted with nitrogen ions prior to doping the source/drain junction. Additional procedures including doping the source/drain junction, depositing nickel onto silicon, and annealing the sample are then performed. Nitrogen ion implantation is found to slow down dopant diffusion and delay transformation from nickel mono-silicide to nickel di-silicide during the high temperature annealing. This process controls dopant transport in shallow source/drain junctions in silicon, but does not improve silicidation of nickel on poly-silicon device structures such as gates. Furthermore, source/drain dopants (particularly Boron) were poorly activated.
Nitrogen has also been used to improve nickel silicidation, in a fifth prior art process described by T. Ohguro et al., IEDM, 453 (1995). This process comprises sputter-depositing nickel onto a single crystal silicon substrate, using a nitrogen containing sputter gas, and then annealing the sample at 400° C. During annealing, nickel reacts with silicon to form nickel mono-silicide. This process forms a uniform nickel mono-silicide thin-film on the surface of the single crystal silicon substrate, without agglomeration. However, this process is difficult to control due to complications, such as non-uniform nitrogen gas pressure within a sputter chamber. This process also does not incorporate nitrogen into the silicon to prevent grain growth in poly-silicon containing substrates.
The prior art processes do not provide a well-controlled way to form a uniform thin nickel mono-silicide film on top of single crystal and polycrystalline silicon. In addition, these processes are difficult to integrate into a nickel salicidation process for high density integrated circuit devices.
There is therefore a need in the art for a nickel salicide process that forms a uniform nickel mono-silicide thin-film on single crystal and polycrystalline silicon underlayers. Ideally, this method should (1) form nickel mono-silicide under a broad range of annealing temperatures; (2) form nickel mono-silicide without further transformation to higher resistivity nickel di-silicide; (3) restrict grain growth in nickel mono-silicide to prevent thin-film agglomeration; and (4) restrict silicide enhanced grain growth in poly-silicon to prevent layer inversion. This method should also be compatible with standard processing of source/drain contacts, gate electrodes, and interconnects in high density integrated circuit devices.